1. Field of the Invention
The present invention relates to an ion implantation system used for manufacturing semiconductor devices. More particularly, it relates to a cooling of an ion target mechanism in the ion implantation system using a high ion dosage.
2. Description of the Related Art
There is an urgent need for a high-speed throughput in an ion implantation (injection) process. The throughput speed is generally defined by the following times: an ion implantation time T.sub.I , a handling time T.sub.H for exchanging a target disk having semiconductor wafers thereon and mounted on a mechanism, and a time T.sub.P for pumping a vacuum chamber. An improvement of the throughput speed can be achieved by shortening these times.
The ion implantation time T.sub.I can be reduced by increasing an ion beam current from approximately 10 mA to 30 mA, which will provide a high ion dosage, approximately 10.sup.15 cm.sup.-2 to 10.sup.16 cm.sup.-2. On the other hand, the reduction of the handling time T.sub.H and the pumping time T.sub.P can be realized by applying a method in which dual end stations are used; a method in which a system has two end stations and the wafer handling and the vacuum pumping for one end station are carried out during the ion implantation of the other end station. Another method for reducing the pumping time T.sub.P is a direct-exchange of the semiconductor wafers.
The dual end station method, however, requires the use of a bulky facility, and the direct wafer exchange method in the vacuum chamber involves a difficult operation, and the quality of a semiconductor wafer may be reduced because dust may be generated from movable portion of the target disk in the vacuum chamber. Therefore, a target disk exchange method has been proposed ("A high-throughput mechanically scanned target chamber", G. Ryding and A. Armstrong, on pp. 319 to 325, "Nuclear Instruments and Methods", No. 189, by North Holland Publishing Company, 1981).
The prior art ion implantation system using the disk exchangeable target mechanism has a low implantation reliability due to heat generated during subjecting the semiconductor wafers on the disk to high ion dosages. This will be described in more detail with reference to a specific example.